Hey I'm guessing by Calibre CLI you mean to do something like manage your book collection, but if all you want to do is convert books, Calibre does come with a CLI called `ebook-convert`. It works with many file formats like epub, pdf, azw3, mobi and more.
Indeed it does. I know that I am not entitled to dictate Calibre's direction, but it would be nice if these tools were cleanly broken out with minimal dependencies. If I recall correctly, the Calibre developers are not interested in this and have stated it explicitly and the only real alternative I have heard of is ebook-converter [1] which simply lifted the relevant portions of code out of Calibre.
I know it was not a criticism but I would like to point out that Calibre seems to be developed also entirely by one person [0]. I understand why they would be hesitant to make big changes that would generate more work for them that would not be seen by most (GUI) users.
Example: `ebook-convert file.epub file.azw3`